The treatment and disposal of sewage is universally recognized as an essential public service required in every community. The purpose of this service is to maintain an acceptable standard of sanitation and to insure maximum protection. But with the best of intentions maximum protection cannot always be maintained, primarily because of the population growth. As an example, a typical sewage treatment plant designed and built to handle a maximum of 500,000 gallons per day approximately 14 years ago now is compelled to operate at nearly 3 times its designed capacity or at a present rate of some 1.4 million gallons per day. A system once considered safe, 14 years ago, can no longer be expected to be operating at 3 times capacity safely. Thus, the effluent discharged from a typical sewage treatment plant is not harmless and, more realistically, is in fact unsatisfactorily treated. Thus, objectionable and even dangerous sewage effluent is being discharged into streams, rivers, lakes and oceans. The problem then presented is the operation of such a typical sewage treatment plant to be prohibited, especially when it is understood that the cost of such a plant 14 years ago was a quarter of a million dollars ($250,000) and the replacement cost of an identical plant today is approximately three quarters of a million dollars ($750,000). Obviously, small communities are committed to what they have. Hence, there is a compelling need to increase the sewage treating capacity of existing, overburdened sewage treatment plants at a reasonable price and in an acceptable manner.
Further, if the quality of the environment is to be upgraded, less costly and more efficient means of sewage treatment techniques must be introduced even in new or in the planning stage of new sewage plants.
Since water is used as a carrier for wastes, it constitutes by far the largest percent of the volume of sewage. The average domestic sewage is composed of approximately 99.9% water and 0.1% solid matter although these numbers may vary considerably from time to time and place to place. While waste matters or impurities of any kind present in sewage constitute only a small percentage of the total volume, their objectionable characteristics, their nature and significance must be understood. The solids are generally classified as organic solids and inorganic solids.
Organic solids: are wastes derived from plant or animal life and are termed organic matter. Organic matter are proteins, carbohydrates and fats. Because these organic solids are largely unstable they will putrefy to produce objectionable odors and create a health hazard. Their removal and stabilization is, therefore, the primary objective of sewage treatment.
Inorganic solids: the mineral present in the water supply, together with the sand, silt and other mineral matter that may find its way into sewers makes up the inorganic matter in sewage. Unless large amounts of these solids are present they do not normally present a treatment problem. Grit removal units are included in most conventional plants to remove suspended inorganic material thus preventing it from entering other plant units.
Referring again to the organic solids or matter, great numbers of bacteria are present in raw sewage. Some bacteria present are unwanted and harmful due to their ability to produce disease, such bacteria are referred to as pathogenic bacteria. Other bacteria present are harmless and in fact are helpful and useful; these are generally referred to as anaerobic bacteria and aerobic bacteria. Anaerobic bacteria are bacteria that thrive in the absence of free oxygen or air and these bacteria perform a vital role in sewage treatment by breaking down organic matter or solids in sewage and hence are utilized in sludge digestion processes. Aerobic bacteria are bacteria that require free oxygen for their life processes and these bacteria are particularly useful in the treatment of sewage due to their ability to oxidize and purify the sewage. More particularly, such aerobic bacteria consume, digest or burn the solid organic matter through the bio-chemical process wherein such aerobic bacteria or organisms in the presence of oxygen convert the organic matter or solids to a more stable form or compound thus preventing such organic matter from decomposing and putrefying.